The contemporary telecommunications systems are increasingly digital in nature and the proliferation of microprocessors has resulted in distributed processing with the systems. In order to take advantage of these developments, the systems tend to be modularized with the modules being interconnected by data links. The information transferred on these links is controlled by various protocols which may be either bit-oriented or byte-oriented.
Bit-oriented protocols comprise the likes of the Synchronous Data Link Control (SDLC), the High Level Data Link Control (HDLC) and the Advanced Data Communications Control Procedure (ADCCP) protocol. These bit-oriented protocols assign specific meaning to the individual bits of each field of the data stream. All communications in such systems are in the form of frames of uniform format and each frame comprises a number of fields each having a definite location and precise meaning.
In byte-oriented protocols, information is transferred in the form of a block of data which may consist of sync characters, an address, control characters, an information field and an error checking code. Special block control characters are used to effect orderly operation of the data link. Once a communications channel is established and the transmitter sends one block of data, it stops and waits for an acknowledgement (ACK) signal before sending another block. The receiver, having acquired the block of data, checks for errors and then sends a positive acknowledgement (PACK) control character to the transmitter indicating that the block is correct or a negative acknowledgement (NACK) control character to indicate an error. On reception of the NACK control signal, the transmitter may retransmit the block of data or take other corrective or maintenance action. Examples of such send-and-wait protocols are the Binary Synchronous Communications (Bisync) protocol and the DS-30 protocol. A description of the latter may be found in U.S. Pat. No. 4,213,201 issued to R. Gagnier et al. and assigned to the present assignee, especially at column 11, lines 33 et seq.
A serious drawback of send-and-wait or compelled protocols is that they are limited to half-duplex (two-way alternate) operation. It is therefore an object of this invention to provide a send-and-wait protocol which provides full-duplex operation.